1. Field of the Invention
This invention relates to an outline image forming method which constitutes one of many image forming processes relying upon the electronic photo-copying technology. More specifically, it relates to a method for the formation of outline images corresponding to peripheral outlines along the outside edge of positive images of a document.
2. Description of the Prior Art
Generally speaking, the peripheral outline of an image is, in practice, full of necessary information thereabout and represents sufficiently the characterizing features of the given image, thus playing among others a most important role in the judgement of the latter.
The so-called outline image means that a peripheral outline is picked up from a generally full and positive documentary image and is devoid of intermediate tones or solid representations, thus being most effective for the identification of the practical image and for pattern recognization thereof.
As an example, a complex color image pattern may be desirable to obtain by execution of successive copying operations to form a blank pattern encircled by a color outline, or to prepare a blank pattern for later producing differently colored local image areas contained therein.
It should be noted that the present assignee company has already proposed to an outline image-forming process, wherein, in a mono-component type toner developing method using conductive toner to develop static latent images, process between the material to be subjected to developing and the carrier for the conductive toner, a d.c.-bias voltage is impressed at a medium potential ranged between the maximum and minimum surface potential of said material and having an opposite polarity to the static latent image charge, for extracting only the outline configuration from the latent image on said material to be developed (refer to Japanese Open Patent Specification, unexamined, Sho-51-134635).
It should be further noted, however, that there is a considerable drawback in the above-mentioned prior art improvement. In this proposed process, indeed, the developed marginal outline image is only a negative one, because of the fact that the marginal outline of the static latent image is also negative and the conductive toner will be deposited onto a substantial part of the latent image other than the marginal outline thereof, by virtue of higher potential difference at the substantive part of the latent image on the drum. In practice, however, the wanted marginal outline should preferably be in black and thus positive. Therefore, the thus formed negative outline image must generally be subjected to a further reproducing step using the reverse development principle, which represents naturally a grave and troublesome drawback inherent in the above known process.